Draggable separator between to-read view and core view.
Core panel can now be resized up and down

The "circle" core is gone, and now we have two vertical panels for the
to-read list and the core, with a separator between them that can be
dragged to resize them at will.
Fixes a bug with the clipping path, now works in Chrome as well
Implements the draggable pane for the fringe

The fringe itself can now be dragged to change how big the fringe area
is compared to the to-read and core views.
Final fixes to merge the new layout with Antoine's refactoring